In the production of vinyl halide polymers, it is common practice to stop the polymerization reaction by which these polymers are produced at a stage prior to that at which the reaction would ordinarily come to a halt in order to obtain stable uniform products having the desired porosity, melt viscosity, and other properties. Chain termination is usually effected by incorporating in the polymerization mixture a compound that will act as a chain-terminating or shortstopping agent for the polymerization reaction without adversely affecting the color of the vinyl halide polymer. The chain-terminating agent also serves to prevent polymerization of recovered monomer and to prevent degradation of the polymer when it is heated during the monomer-removal step and/or the drying step of the process.
One of the most commonly-used chain-terminating agents in the commercial production of vinyl chloride polymers is 2,6-di-tertiary butyl-p-cresol (BHT). 2,6-Di-tertiary butyl-p-cresol not only acts as an efficient chain-terminating agent during the final processing steps and yields polymers having the desired combination of physical properties but it also provides some improvement in the long term heat stability of the finished resin. The use of water-soluble phenols in the amount of 0.1% to 0.5%, based on the weight of the polymer, as the chain-terminating agents in the production of vinyl halide polymers was disclosed by Hoertz in U.S. Pat. No. 2,662,867.
Antimony organic sulfur- and/or oxygen-containing compounds, such as antimony mercaptoacid esters, antimony alkyl mercaptides, and antimony carboxylates, are known to be effective heat stabilizers for vinyl halide resin compositions. The preparation of these organic antimony compounds and their use as stabilizers are disclosed in a number of patents including U.S. Pat. Nos. 2,680,726, 2,684,956, 3,340,285, 3,347,821, 3,399,220, 3,466,281, and 3,530,158. In a process that is used commercially for the production of these antimony compounds, antimony oxide is reacted with a stoichiometrically equivalent amount of an acidic organic sulfur- and/or oxygen-containing compound, for example, a mercaptoacid ester, an alkyl mercaptan, or a monocarboxylic acid. The preparation of antimony mercaptoacid esters by this procedure is shown in the following equation: EQU Sb.sub.2 O.sub.3 +6HSRCOOR'.fwdarw.2Sb(SRCOOR').sub.3 +3H.sub.2 O
When 3 moles of mercaptoacid ester are used per mole of antimony, as is shown in the equation, the reaction generally does not go to completion, and the liquid products obtained after filtration, which contain unreacted antimony oxide, are hazy and unstable. The instability of the antimony organic sulfur- and/or oxygen-containing compounds, their ineffectiveness in preventing the early discoloration of vinyl halide resin compositions, and their tendency to exude from resinous compositions have limited their use as stabilizers for vinyl halide resin compositions.
Attempts have been made to overcome these deficiencies by adding various materials to the antimony compounds, but the resulting stabilizers and resinous compositions containing these stabilizers have not proven to be entirely satisfactory. The materials that have been used to improve the properties of antimony compounds as stabilizers for vinyl halide resin compositions include alkali metal or alkaline earth metal salts of carboxylic acids (U.S. Pat. No. 3,887,508), alkali metal or alkaline earth metal salts of mercaptans or mercaptoacids (U.S. Pat. No. 4,115,352), phenols (U.S. Pat. Nos. 4,029,618 and U.S. 4,158,640), and tri(alkali metal phosphates (U.S. Pat. No. 3,919,168). Among the most useful of the antimony stabilizer compositions disclosed in the prior art are those disclosed in U.S. Pat. No. 4,029,618, which consist essentially of an antimony organic sulfur-containing compound and 1 to 10% based on the weight of the antimony compound, of an ortho-dihydric phenol. In copending application Ser. No. 144,141, which was filed on Apr. 28, 1980 now U.S. Pat. No. 4,336,168, Hoch et al. disclosed shelf-stable liquid antimony stabilizers for vinyl halide resin compositions that comprise a liquid antimony compound, such as an antimony mercaptoacid ester having the formula Sb(SRCOOR').sub.3, an antimony mercaptide having the formula Sb(SR').sub.3, or an antimony carboxylate having the formula Sb(OOCR"').sub.3 and a stabilizing amount of an acidic organic compound that is a mercaptoacid ester, a mercaptan, a mercaptoacid, a mercaptoalcohol, a monocarboxylic acid, or a mixture thereof.